I really wanted the T-Mobile plan, which has unlimited data in theory, but slow after a tier that you pay for. In this case, for you, 5 GB is $70/month. But then you get some free international roaming, which is slow, but could be nice. And tethering works AFAIK, on the US service.

Another important note here: you said you switched your SIM on your old phone. That won't work on a new subsidized one from AT&T for two years, and after two years, you'll still need their mysterious blessing.